Application of Microprobe Analysis to the Reconstruction and Characterization of Dendritic Structures

The electron probe microanalyzer (EPMA or “microprobe”) is a powerful tool for nondestructive chemical analysis of solid materials. The work presented in this article proofs the concept of reconstructing three-dimensional (3-D) dendritic structures in steel based on 5 two-dimensional (2-D) EPMA concentration maps. The EPMA measurements are focused on the concentration distribution of Mn, which has a distinct microsegregation tendency in steel. Because the concentration maps must be taken from different depths of the investigated sample, serial sectioning of the sample is required for each microprobe measurement. These measured concentration maps are processed with a commercial software tool to smooth and to merge the maps, as well as to consider the temperature gradient that occurred during solidification on the microsegregation pattern. Afterward, the concentration maps are stored in a 3-D array, and the neighboring array entries with the same predefined threshold concentration value are connected with surfaces to build a 3-D dendritic structure. Because the concentration of certain alloying elements in the solid phase increases during the solidification process, it is possible to visualize the dendritic growth by increasing the solid fraction. Finally, a simple correlation was used to relate the specific surface area to the solid volume fraction of the dendrites. The obtained 3-D structures can be used for subsequent investigations in finite-element (FE) or computational fluid dynamics (CFD) simulation tools.